The fields of nanoscience and nanotechnology generally concern the synthesis, fabrication and use of nanoelements and nanostructures at atomic, molecular and supramolecular levels. The nanosize of these elements and structures offers significant potential for research and applications across the scientific disciplines, including materials science, physics, chemistry, computer science, engineering and biology. Biological processes and methods, for example, are expected to be developed based entirely on nanoelements and their assembly into nanostructures. Other applications include developing nanodevices for use in semiconductors, electronics, photonics, optics, materials and medicine.
One class of nanoelements that has garnered considerable interest consists of carbon nanotubes. P. Teredesai et al., “Pressure-Induced Reversible Transformation in Single-Walled Carbon Nanotube Bundles Studied by Raman Spectroscopy,” Chem. Phy. Let., 319, 296-302 (2000). A carbon nanotube has a diameter on the order of nanometers and can be several micrometers in length. These nanoelements feature concentrically arranged carbon hexagons. Carbon nanotubes can behave as metals or semiconductors depending on their chirality and physical geometry.
Although carbon nanotubes have been assembled into different nanostructures, only limited nanotools and fabrication methods for their assembly have been developed. One obstacle has been the manipulation of individual nanoelements, which is often inefficient and tedious. This problem is particularly challenging when assembling complex nanostructures that require selecting and ordering millions of nanoelements across a large area.
Nanostructure assembly has focused on dispersing and manipulating nanoelements using atomic force or scanning tunneling microscopic methods. Although these methods are useful for fabricating simple nanodevices, neither is practical when selecting and patterning, for example, millions of nanoelements for more complex structures. As an alternative, lithographic methods have been developed to modify substrates used for assembling nanoelements. Examples of these lithographic methods include, but are not limited to, electron-beam, ion-beam, extreme ultraviolet and soft lithographies. These methods, however, remain incapable of manipulating individual nanoelements. The development of nanomachines or “nanoassemblers” which are programmed and used to order nanoelements for their assembly holds promise, although there have been few practical advancements with these machines.
Self-assembly is a method for nanodevice fabrication that does not require nanoelements to be individually manipulated. In self-assembly, nanoelements are designed to naturally organize into patterns by atomic, molecular and supramolecular particle interactions. Self-assembled monolayers, for example, are formed by the spontaneous arrangement of molecules into monomolecular layered structures. These structures can be stabilized by van der Waals forces or other forms of noncovalent bonding. Self-assembled monolayers, however, have been problematic when used to transfer nanoelements from one nanosubstrate to a recipient substrate. Although particle interactions can be modified to affect their transport, optical and electrical properties, controlling nanoelement orientation is also a challenge in self-assembly methods. Similarly, nanoscience has been incapable of manipulating particle interactions to reproducibly assemble hundreds of nanodevices.
The advancement of nanotechnology requires millions of nanoelements to be conveniently selected and simultaneously assembled. Nanostructure assembly also requires that nanoelements be ordered across a large area. Previously available methods such as those mentioned above have yet to meet these requirements.
Carbon nanotubes have shown promise as next generation switches or interconnects in electronic applications due to their unique electronic properties. The hindrance to the realization of electronic circuits with carbon nanotubes is the difficulty in positioning and contacting them in a controlled way on a large scale. One method that has been reported in the literature involves spinning a carbon nanotube suspension over a wafer. This produces a random dispersion of carbon nanotubes onto substrates which lack alignment. Another approach utilizes catalytic growth of nanotubes using catalyst particles. Self-assembly on chemically modified surfaces also has been used. These techniques, however, are not suitable for large scale controlled assembly of carbon nanotubes and other nanoelements.
Recently, assembly of carbon nanotubes and nanoparticles on patterned surfaces has been accomplished using electric fields. This process depends on the field generated by the voltage supplied to wires. Assembly of nanoelements works well when using microscale wires to generate the field. However, when the wires are reduced from diameters in the micron range to the nanometer range, their resistance increases by as much as two orders of magnitude. Previous attempts to direct the assembly process using DC electrophoresis with nanowires have failed.